1. What is carbon steel?
Based on its chemical composition, steel can be categorized into two main types: carbon steel and alloy steel.
Carbon steel is further divided into:
- Low carbon steel – with a carbon content of less than 0.25%
- Medium Carbon Steel – with a carbon content of 0.25% to 0.6%
- High carbon steel – with a carbon content greater than 0.6%
1. Low carbon steel
Low carbon steel, also known as carbon steel, is a type of carbon steel with a carbon content of less than 0.25%. It is referred to as mild steel because of its low strength and hardness.
This category includes most common carbon structural steels and a portion of high-quality carbon structural steels. It is mainly used for engineering structural components without heat treatment. Some undergo carburizing and other heat treatments for wear-resistant mechanical parts.
2. Medium Carbon Steel
Medium carbon steel has good thermal processing and cutting performance, but its weldability is low. Its strength and hardness are superior to those of low carbon steel, but its plasticity and toughness are inferior.
It can be used directly without heat treatment or after heat treatment. Quenched and tempered medium carbon steel has excellent comprehensive mechanical properties. The highest achievable hardness is approximately HRC55 (HB538), with tensile strength of 600 to 1100MPa.
Therefore, medium carbon steel is widely used at moderate strength levels not only as a construction material but also extensively in manufacturing various mechanical parts.
3. High carbon steel
High carbon steel, often referred to as tool steel, has a carbon content ranging from 0.60% to 1.70%. It can be hardened and tempered, but its weldability is poor.
Tools such as hammers and crowbars are made from steel with a carbon content of 0.75%; Cutting tools such as drills, taps and reamers are made from steel with a carbon content of 0.90% to 1.00%.
2. Comparison of weldability between low carbon steel and high carbon steel
The weldability of steel depends mainly on its chemical composition, with carbon being the most influential element.
In other words, the carbon content determines the weldability of the metal. Most other alloying elements in steel also make welding difficult, but their impact is generally much less significant than carbon.
Low carbon steel generally has good weldability and does not require special process measures.
However, when working with thick sheets, low temperatures or high demands, it is necessary to weld with alkaline electrodes and carry out adequate preheating.
When the carbon and sulfur content in low-carbon steel is close to the upper limit, not only high-quality low-hydrogen electrodes and preheating and postheating measures should be used, but the groove shape should also be selected appropriately, and the melting rate should be reduced to avoid hot cracking.
Medium carbon steel tends to develop cold cracks when welded. The higher the carbon content, the greater the tendency for hardening in the heat-affected zone and the greater the probability of cold cracking, leading to worse weldability.
As the carbon content in the base material increases, the carbon content in the weld metal also increases. Combined with the adverse effect of sulfur, hot cracks can easily form in the weld.
Therefore, when welding medium carbon steel, electrodes with good crack resistance should be used, and measures such as preheating and postheating should be taken to reduce the tendency of cracking.
When welding high carbon steel, due to its high carbon content, significant welding stresses are produced. The heat-affected zone tends to harden and develop cold cracks, and the weld is more prone to hot cracking.
High carbon steel is more likely to develop hot cracks than medium carbon steel during welding, making it the worst in terms of weldability.
Therefore, it is generally not used in welding structures and is only used for repair welding or coating of castings. After welding, the weldment must be tempered to eliminate stresses, stabilize the structure, prevent cracking and improve weld performance.
3. Welding of medium carbon steel
Medium carbon steel refers to carbon steel with a carbon content of 0.25% to 0.60%, which includes high-quality carbon structural steel grades such as 30, 35, 45, 50, 55 and grades of cast carbon steel, such as ZG230-450, ZG270 -500, ZG310-570 and ZG340-640.
Due to the higher carbon content in medium carbon steel compared to low carbon steel, its weldability is lower. When the carbon mass fraction is close to 0.30% and the manganese content is not high, the weldability is still good, but as the carbon content increases, the weldability gradually worsens.
When the mass fraction of carbon reaches about 0.50%, the weldability worsens significantly.
(1) Common problems in welding medium carbon steel
Problems that may occur when welding medium carbon steel are as follows:
- Cold cracking problem
Due to the high carbon content in steel, the heat-affected zone can easily produce a hard and brittle martensite structure during welding, thus leading to cold cracking.
If unsuitable welding materials are used or if the welding process is not formulated correctly, cold cracks can also easily occur in the weld.
- Hot cracking problem
During welding, the original material with high carbon content will melt and introduce carbon into the weld, thereby increasing the carbon content of the weld. Carbon can intensify the effect of sulfur and phosphorus on metals, causing hot cracking.
Therefore, when welding medium carbon steel, hot cracks can easily occur in the weld. This is especially true when the sulfur and phosphorus content in the base material or welding material is not strictly controlled, increasing the likelihood of hot cracking.
Additionally, the high carbon content in steel can also increase the weld's tendency to produce CO gas pores.
(2) Medium carbon steel welding techniques
Due to the propensity of medium carbon steel to form defects such as cold and hot cracks when welded, special technical measures need to be implemented to ensure successful welding.
- Welding methods
Various arc welding methods can be employed for welding medium carbon steel. Because medium carbon steel is commonly used in the production of machine parts, rather than large-scale welding structures, shielded metal arc welding is more often used.
- Welding Materials
To prevent the formation of cold and hot cracks in the weld, low hydrogen electrodes are typically used in shielded metal arc welding. These electrodes not only maintain a low hydrogen content in the weld, but also exhibit desulfurizing and dephosphorizing effects, increasing the plasticity and toughness of the weld.
When the steel has a lower carbon content and the joint has less rigidity, rutile or basic electrodes can be used. However, strict technical measures must be implemented, such as minimizing the melting rate, rigorously preheating the part and controlling the interlayer temperature.
If preheating is not possible, austenitic stainless steel electrodes such as E308L-16 (A102), E308L-15 (A107), E309-16 (A302), E309-15 (A307), E310-16 (A402) , E310- 15 (A407), can be used.
- Preheating and intermediate temperature
Preheating is the most effective technique for preventing cracks when welding medium carbon steel. Preheating not only reduces the joint cooling rate, preventing martensite formation, but also reduces welding stress and accelerates hydrogen diffusion.
In most circumstances, it is necessary to preheat and maintain the temperature of the middle layer.
The selection of preheat and interlayer temperatures depends on the carbon equivalent of the steel, the thickness of the base metal, the rigidity of the structure and the type of electrode.
The preheating temperature can be determined through welding tests, or through the empirical formula T0=550(C-0.12)+0.4δ. In this formula, T0 represents the preheating temperature (℃), C represents the mass fraction of carbon in the base metal to be welded (%) and δ represents the thickness of the steel plate (mm).
The preheating and interlayer temperatures for welding steels 30, 35 and 45 can be referenced in Table 1.
Table 1 Preheating temperature and post-welding tempering temperature for welding carbon steel
| Steel Grade | Welding thickness /mm |
Operation process | Welding Rod Category | Observation | |
| Interim preheating temperature /℃ |
Stress Relief Quenching Temperature /℃ |
||||
| 30 | -25 | >50 | 600-650 | Non Low Hydrogen Type Welding Rod | 1. The heating range on both sides of the groove for local preheating is 150-200mm
2. During the welding process, hammering can be used to reduce welding residual stress. |
| Low Hydrogen Type Welding Rod | |||||
| 35 | 25-50 | >100 | Low hydrogen type | ||
| >150 | Non-Low Hydrogen Type | ||||
| 50-100 | >150 | Low hydrogen type | |||
| 45 | -100 | >200 | Low hydrogen type | ||
- Groove shape
Ideally, the workpiece should have a U- or V-shaped groove to reduce the proportion of base metal melted into the weld. When repairing defects in castings, the excavated groove must have a smooth exterior to minimize the amount of base metal that melts into the weld.
- Welding parameters
Direct current reverse polarity power supply must be used for welding. For multilayer welding, small diameter electrodes, low current and slow welding speed must be used, as the proportion of base metal that melts in the first layer of the weld can reach up to 30%.
- Post-weld heat treatment
After welding, the workpiece should ideally be immediately subjected to a stress-relieving heat treatment. This is especially important for thick weldments, highly rigid structures and weldments operating under dynamic or impact loads.
The temperature for stress-relieved annealing is generally between 600 and 650 degrees Celsius.
If stress relief heat treatment cannot be carried out immediately after welding, post-heating must be carried out, which involves heating slightly above the pre-heating temperature, with a waiting time of approximately 1 hour per 10mm of thickness.
(3) Compilation of typical welding procedures for steel with medium carbon content
(I) Steel 35 and Cast Carbon Steel ZG270-500
The mass fraction of carbon in 35 steel is 0.32% to 0.39%, and that of ZG270-500 cast carbon steel is 0.31% to 0.40%. The carbon equivalent is about 0.45%, so the weldability of this type of steel is acceptable.
However, in the area affected by heat during welding, a hard and brittle martensitic structure can form, which tends to crack. Therefore, certain technical measures must be taken when welding this type of steel.
- Selection of welding materials
When using stick arc welding, if a weld seam of equal strength to that of the original material is required, E5016 (J506) or E5015 (J507) welding rods can be used. If a weld seam of equal strength to that of the base material is not required, welding rods E4316 (J426), E4315 (J427), E4303 (J422), E4310 (J423) etc. can be selected.
For submerged arc welding, HJ430 or HJ431 fluxes and H08MnA or H10Mn2 wires can be selected.
For slag welding, HJ430, HJ431, HJ360 fluxes and H10Mn2, H08Mn2Si, H08Mn2SiA wires can be selected.
- Preheating temperature and interlayer temperature
When welding 35 steel and ZG270-500 cast steel, the typical preheating temperature and interlayer temperature for the welded parts are about 150℃. If the stiffness of the welded parts is relatively large, the preheating temperature and interlayer temperature should be increased to 200-250°C.
The heating range for local preheating is 150-200mm on both sides of the groove.
- Post-weld heat treatment
For welded parts with high thickness, high rigidity or that work under dynamic or impact loads, stress relief annealing must be carried out immediately after welding. The annealing temperature is generally 600-650°C.
For general thickness welded parts, postheating can be used to allow hydrogen diffusion to escape.
The post-heating temperature is generally 200-350°C and the holding time is 2-6 hours depending on the thickness of the welded parts.
(II) Steel 45 and Cast Carbon Steel ZG310-570
The mass fraction of carbon in 45 steel is 0.42% to 0.5%, and that of ZG310-570 cast steel is 0.41% to 0.50%. The carbon equivalent is about 0.56%. This steel has a greater tendency to harden and crack, making its weldability relatively low.
- Selection of welding materials
For electrode arc welding, welding rods with a low hydrogen content should be chosen. If a weld seam of equal strength to that of the base material is required, E5516-G (J556) or E5515-G (J557) welding rods may be used.
If a weld seam of equal strength to the base material is not required, E4316 (J426), E4315 (J427), E5016 (J506), E5015 (J507), E4303 (J422), E4301 (J423) etc. selected.
For submerged arc welding, HJ350 or SJ101 fluxes and H08MnMoA wires can be selected.
- Selection of welding parameters
When welding 45 steel and ZG310-570 cast carbon steel, a lower welding current should be chosen to reduce the melting rate of the weld seam and decrease the amount of carbon transitioning from the parent material to the weld seam.
- Preheating temperature and interlayer temperature
To weld this type of steel, it is advisable to preheat the entire piece to a temperature above 200 ℃.
For T-joints, because they have more heat dissipation directions than butt joints, the cooling speed of the welded joint will increase, increasing the tendency to produce cold cracks.
Therefore, the preheating temperature should be appropriately increased to 250-400°C depending on the thickness of the welded parts.
The interlayer temperature must not be lower than the preheating temperature.
- Post-weld heat treatment
After welding, the welded parts must immediately undergo annealing to relieve stress. The annealing temperature is 600-650°C.
